MIE Field of MSTATUS CSR Is Updated Wrongly

[shetalani]

RISC-V Specification:

• "The MRET, SRET, or URET instructions are used to return from traps in M-mode, S-mode, or U-mode respectively. When executing an xRET instruction, supposing xPP holds the value y, xIE is set to xPIE; the privilege mode is changed to y; xPIE is set to 1; and xPP is set to U (or M if user-mode is not supported)."
• "The MRET instruction is used to return from a trap taken into M-mode. MRET sets the privilege mode according to the values of MPP and MPV in mstatus, as encoded in Table 5.6. MRET then in mstatus sets MPV=0, MPP=0, MIE=MPIE, and MPIE=1, and also sets pc=mepc."

Issue Description:

MIE field of MSTATUS CSR is not updated to take the value of MPIE when MRET instruction is executed and the MPP field has the value 0.

Example:

As shown below, the instruction 32'h30200073 (mret) is decoded at time point t##0 where the current privilege is M and MPP has the value associated with U mode. When it updates MSTATUS value at t##3, it sets all fields correctly except MIE, which stays asserted instead of taking the value of MPIE.

On the other hand, in the opposite scenario, when an exception occurring in U mode is handled, MPIE is not taking the value of MIE as the specification states.

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Product: OneSpin 360 DV-Verify App: RVV Tool's version: 2019.2.2

[silabs-PaulZ]

Hi @shetalani , The User mode was removed when we ported RISCY to CV32E40P. We will add User Mode back in on the follow-on part, CV32E40. I hope to look at the user mode issues at a later date.

Regarding machine mode, I am now seeing the correct behavior. After mret instruction call, the following holds true:

• MIE= MPIE
• MPIE=1

• current_prv_lvl = machine (which is always machine in CV32E40P)

  The following waveforms show that MIE and MPIE are valid for both scenarios of MIE (0,1) before the trap.

MPRV, UIE, and UPIE are tied to 0 in CV32E40P.